The waveform that exists at all points in a power transmission system is ideally a sinusoid of constant frequency. In the US, that frequency is 60 Hz. However, in many parts of the world, the frequency is 50 Hz.
The power transmitted by a power transmission and distribution system is represented by a power vector in the complex plane. The imaginary component of the power vector represents reactive power, while the real component represents the power that actually carries out useful work. The ratio between the real component of the power vector and the magnitude of the power vector is referred to as a “power factor.”
The relative magnitudes of the real and reactive power depend on system loads. These system loads, which can change from time to time, affect a phase relationship between the voltage and current waveform on the transmission line. This phase relationship depends on the imaginary part of the impedance (i.e. the reactance) seen by the transmission line. As these loads increase, the transmission and/or distribution line voltages can change from their desired levels.
In response to changes in the load, one can provide or withdraw reactive VARs to control the voltage level on the network. One known device for carrying out this function is a static VAR compensator. These static VAR compensators use high-voltage thyristors to connect and disconnect a reactive capacitor from the line. The control of these thyristors is critical to avoid damaging them.
The control of the thyristors depends in part on the voltage waveform present on the line. This voltage waveform is subject to electrical disturbances. In some cases, these electrical disturbances are dominated by harmonic content that is, to some extent, predictable. When this is the case, techniques such as that described in U.S. patent application Ser. No. 12/749,390, filed on Mar. 30, 2010 and incorporated herein by reference, can be used to determine when gate current can safely be turned off, and when it should be applied. However, when the electrical disturbances occur at random, the techniques disclosed in the foregoing patent application are less effective.